Abstract: Mode localization is often an unexpected dynamicphenomenon in weakly-coupled symmetric structure, and it arises from smallimperfections (less than 5\%) which perturb the symmetry. Suchimperfections typically result from random manufacturing or assemblyimprecision. Mode localization prediction is an important problem in T-Tailstructure design because some drastic localized vibration phenomena occurduring ground vibration test of T-Tail aircraft. Mode localization isdependent not only on mistuning, but also on coupling intensity. In thispaper, we define the degree of mode localization with the peak amplituderatio and the coupling intensity with stiffness ratio of horizontalstabilizer to fin in T-Tail structure. The influence of mass mistuning onmode localization and frequency loci veering of T-Tail structure is studied.Based on the mass mistuning model, four different criterions are proposed topredict the occurrence of mode localization in T-Tail structure from fourview points, which are the peak amplitude ratio, coupling mistuned ratio,ordinary eigenvalue perturbation method and eigenvalue perturbation methodfor nearly equal frequencies, respectively. The numerical simulation resultsfor a T-Tail structure model indicate that mode localization is most likelyto occur in T-Tail structure consisting of weakly coupledsubstructures---horizontal stabilizer and fin. Moreover, when localizationoccurs, the first two bending vibration modes in T-Tail structure are proneto localization, and only one of the two mode frequencies is changed byparameter mistuning to induce frequency loci veering phenomena. The resultsof this study also demonstrate the feasibility and effectiveness of theproposed four predicting criterions of mode localization, and thus provide areference solution to the prediction and design of mode localization of thiskind structure.